HPLC technology is widely used to detect and identify different components contained in a test sample or specimen material. A typical HPLC instrument has a high pressure pump for forcing a suitable solvent, via capillary lines, at a constant flow rate serially through a sampler, a separation column and a UV or other type detector. Periodically, a small quantity of the test sample will be injected via the sampler into this flowing solvent stream to travel then somewhat as a slug with the solvent stream into the separation column. The separation column contains an absorbent reactive anticipated sub phase components in the test sample. Thus, the different sub phases pass through the detector at different rates, producing separated flow through the detector suited for identification and quantitative analysis.
Different separation columns are commercially available for different testing needs, being of different lengths and/or diameters, and/or being packed with different absorbents and/or to different degrees of compactness.
A typical separation column consists of a rigid tube having couplings secured relative to its opposite ends, where each coupling has an internally threaded end cavity and capillary line adapter. A tubular exteriorly threaded fitting, snugged over an end of a capillary line, can be tightening into the column end coupling for establishing a reliably liquid-tight connection therewith. This allows the column to be easily connected into the HPLC instrument in a series flow circuit with the capillary line between the sampler and detector.
The column tube bore has a substantially uniform diameter (such as between 0.5 mm to 7 mm) machined and polished to a high finish. The tube bore is densely packed with the absorbent of micron sized particles, and sintered or mesh filters made of steel, titanium or PEEK close or cap at both tube ends. A sealing member secured or pressed onto the tube end generally holds the filter in place across the bore end.
The column provides high resistance to the liquid test sample mixture flowing through it, requiring pumped liquid pressures up to 5,000 psi, although only a small quantity of liquid sample is used (a few mcls) for each test. To withstand blow-out forces due to these high operating bore pressures, each end coupling must be mechanically secured relative to the tube.
To achieve this solid mechanical connection, some end couplings are threaded onto the tube ends while others utilize a stop ring compressed on the tube exterior for holding one component of a conventional threaded two-component fitting connection. However, thread machining adds costs, as does machining needed to form the multi-faced fittings (hex shapes or the like) for defining faces to accept a wrench or other tool for tightening the threaded components together.
Several problems can occur with separation columns having the above noted end couplings. For example, the column must have an identification label secured to it to identify its parameters. Such labels commonly have been taped around and onto the mid-portion the bore tube between the end couplings. This limits the shortest length column to several centimeters, as the smallest label and each end coupling might extends about a centimeter along the tube length, and label cannot cover the end couplings.
Further, it is desirable during certain testing procedures to maintain the column at a specific temperature, and present procedures place the column then on a flat heating or cooling plate held at that desired temperature. However, as the end couplings project radially beyond the tube exterior, only they contact the thermal plate (while the column mid-section is gapped from the thermal plate), meaning temperature uniformity along the column length remains uncertain.